Fresh from the Mouth of AT&T, who looks like his Lord of the Rings counterpart, is the reason AT&T blocked 4chan. It was, as suspected, blamed on a DDoS attack from that IP address. Update: 4chan's moot confirms their account:

It's indisputable at this point that AT&T blocked 4chan—what's 4chan, you ask?—the…
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Beginning Friday, an AT&T customer was impacted by a denial-of-service attack stemming from IP addresses connected to img.4chan.org. To prevent this attack from disrupting service for the impacted AT&T customer, and to prevent the attack from spreading to impact to our other customers, AT&T temporarily blocked access to the IP addresses in question for our customers. This action was in no way related to the content at img.4chan.org; our focus was on protecting our customers from malicious traffic.

Overnight Sunday, after we determined the denial-of-service threat no longer existed, AT&T removed the block on the IP addresses in question. We will continue to monitor for denial-of-service activity and any malicious traffic to protect our customers.

I'm sure that won't stop the 4chan retaliation, but it's good to know censorship hasn't taken hold at AT&T. Yet.

Unfortunately, as an unintended consequence of the method used [to filter a DDoS attack against 4chan], some Internet users received errant traffic from one of our network switches. A handful happened to be AT&T customers.

In response, AT&T filtered all traffic to and from our img.4chan.org IPs (which serve /b/ & /r9k/) for their entire network, instead of only the affected customers. AT&T did not contact us prior to implementing the block. Here is their statement regarding the matter.

In the end, this wasn't a sinister act of censorship, but rather a bit of a mistake and a poorly executed, disproportionate response on AT&T's part. Whoever pulled the trigger on blackholing the site probably didn't anticipate [nor intend] the consequences of doing so.